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Electric car range testing explained: How far do they go?

These tests determine MPGe and range without the car ever hitting the road.

An email from Peter A. of Vancouver, Washington, posed a practical question: How do you know an electrified car, like the Bolt, Leaf or Model 3, will really deliver the range it advertises? And, in his case, he's looking for a plug-in hybrid, like a Toyota Prius or Hyundai Ioniq, which is an even thornier matter.

First, understand MPGe, the miles per gallon equivalent: It's a metric that assumes a gallon of gas contains 115,000 BTUs of energy and equates that to 33.7 kilowatt hours (kWh) of electrical energy. 

Now playing: Watch this: How they test electric cars to establish range and MPGe

For example, a 2019 Nissan Leaf Plus has a 68kWh battery. Divide that by 33.7kWh and you get about two "gallons" of electricity, which doesn't sound like much until you know that EV motors are about 85% efficient compared to about 25% for gas engines. That allows a Leaf to get an average of 108 miles from each of its electric "gallons," or 108 MPGe.

On a plug-in hybrid, like a Hyundai Ioniq, you'll see both MPGe and traditional gas MPG ratings, since plug-in cars have two powerplants. MPGe shows efficiency when the car is running in pure electric mode, while MPG reflects driving on the gas engine alone with as a hybrid of that and electric power. 

Testing pure electric range is similar to the EPA test cycles for gasoline economy, but is based on something called the SAE J1634 test specification for electric cars:

  • The car is fully charged, then parked overnight.
  • The car is mounted on a dynamometer, not tested on the road. 
  • Simulated city or highway driving is done on the dyno until the battery is spent and the miles driven are noted. 
  • The car is recharged from an AC source, not a DC fast charger. The energy required for full recharge is noted.
  • That charge amount is divided by the miles driven to calculate gross MPGe.  
  • Gross MPGe is multiplied by 0.7 to account for real-world factors like wind, terrain and HVAC use. 

But, depending where you live, you may want to give the EPA range rating a further haircut: A recent AAA test of five pure electric cars found that heater use in 20-degree weather resulted in an average 40% decrease of range and MPGe, while use of air conditioning in 95-degree weather resulted in about a 17% decrease.